Currently, radio communication systems such as cell phone systems and wireless metropolitan area networks (MANs) are widely used. Meanwhile, in the field of radio communications, lively discussions have been taking place about the next-generation communication technologies so as to increase the transmission speed and transmission capacity.
In radio communications, one radio communication apparatus may transmit a control signal to another radio communication apparatus. Information to be transmitted as a control signal includes information (e.g., information indicating a radio resource used for data transmission and a modulation coding scheme) which the other radio communication apparatus refers to so as to receive data transmitted by the radio communication apparatus. The information to be transmitted as a control signal also includes information (e.g., information indicating a radio resource to be used for data transmission and a specified modulation coding scheme) which the other radio communication apparatus refers to so as to transmit data to the radio communication apparatus.
In radio communication systems such as LTE (Long Term Evolution) system and LTE-A (LTE-Advanced system), for example, a radio resource area (search space) in which a radio communication apparatus at the receiving end monitors whether there is a control signal is defined in each radio subframe in the radio downlink. There are two types of search spaces: the common search space for control signals that are commonly referred to by a plurality of radio communication apparatuses, and the UE (User Equipment)-specific search space for control signals that are referred to by a particular radio communication apparatus. A radio communication apparatus at the transmitting end transmits a control signal using radio resources in the search space corresponding to the destination of the control signal. The radio communication apparatus at the receiving end monitors the common search space and the UE-specific search space corresponding to the radio communication apparatus at the receiving end, and thereby detects the control signal. Since such search spaces are defined, it is possible to limit the area to be monitored by each radio communication apparatus, and therefore to reduce the signal processing workload for detecting a control signal addressed to the radio communication apparatus.
For instance, in the LTE system, the location of the UE-specific search space may be adjusted in accordance with the identifier of the radio communication apparatus at the receiving end and the number of the subframe (a time unit for scheduling transmission of radio signals) (see, 3rd Generation Partnership Project, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer procedures”, 3GPP TS 36.213 V9.0.1, 2009-12, Section 9.1.1, for example). In this case, each of the radio communication apparatuses at the transmitting end and the receiving end can calculate the location of the UE-specific search space on the basis of a predetermined algorithm. It is to be noted that, in radio communications, the amount of control signals to be transmitted is not always constant. Therefore, in order to use the radio resources effectively, a plurality of search spaces may be set to overlap one another.
Further, as for the LTE-A system, for example, radio communication using a plurality of frequency bands in parallel is under study. Each of a plurality of frequency bands is often referred to as a component carrier. A method of transmitting a control signal and data corresponding thereto in different frequency bands in the case where a plurality of frequency bands are used is also under study. This method of transmitting a control signal is often referred to as cross-carrier scheduling. With cross-carrier scheduling, it is possible to transmit control signals for a plurality of frequency bands in a single frequency band (see, 3rd Generation Partnership Project, “PDCCH monitoring set”, R1-094571, 3GPP TSG RAN WG1 Meeting #59, 2009-11, and 3rd Generation Partnership Project, “Considerations on issues of UE Component Carrier set”, R1-094676, 3GPP TSG RAN WG1 Meeting #59, 2009-11).
In the case where control signals are transmitted using a smaller number of frequency bands than the number of frequency bands to be used for data communication, the amount of control signals to be transmitted in a single search space is increased. Meanwhile, as mentioned above, there are overlaps between a plurality of search spaces. Therefore, depending on the usage of the other search spaces, the amount of the practically available radio resources may be reduced, which may result in failing to reserve a sufficient amount of radio resources corresponding to the amount of the control signals to be transmitted.